Heat exchangers are in wide use in industry. One application for heat exchangers is the vaporization of liquefied natural gas (LNG). Systems are known for adding heat to liquefied natural gas to convert it to a gas state. One type of LNG evaporator is a so-called submerged combustion vaporizer (SCV). An SCV generally has a water bath tank in which is submerged a vaporization coil. Liquefied LNG is supplied to the vaporization coil from outside the SCV, runs through the coil and is evaporated inside the coil, and exits the SCV as a gas. To accomplish this, heat needs to be continually added to the water bath.
One way the heat can be added is that the SCV contains a partially submerged fan-driven combustion device having a specially designed burner which will produce hot flue gases that are conveyed via distributor ductwork and sparger assemblies into the water bath below the LNG vaporization coil. The sparger assemblies direct heat into the water bath both by surface convection on the outside metal walls and conductive heat transfer via direct contact of hot flue gases with surrounding water. Thus, heat transferred to the water bath is subsequently transferred to the outside metal wall surface of the LNG vaporization coil submerged with the water bath.
Some SCVs have an adjustable operating range of heat addition. That is, there will be a 100% design operating condition, whereby the fan and/or fuel gas burn rate can be reduced continuously to a lower level as necessary to match the heat input requirements to produce/maintain the desired LNG vaporization rate. However, there is a lower limit to which the SCV output may be reduced without reaching mechanical/process constraints with the equipment. That is, it is difficult to operate a SCV below a certain minimum level because turndown capability is fixed by factors such as fan performance characteristics and flame stability under various air/fuel ratios. This turndown capability characteristic of SCVs is discussed in more detail below.
Another method for adding heat to a water bath is the use of an atmospheric heating tower. Atmospheric cooling towers are well known, and it has been found that where it is desired to heat a fluid, rather than cool it, so long as the atmospheric temperature is greater than the supply temperature to the tower, it is possible to use atmospheric towers that are configured generally like cooling towers but operated in a fashion so that they will actually heat the water supplied to the tower and provide an output warmer than the input.
Operation of SCV, and operation of heating towers, often occur in areas where the ambient temperature will change both during the course of the day and evening, and also seasonally. It would be desirable to have a system and method that could control an SCV and/or heating tower in such a way as to provide the efficient and effective vaporization of LNG.